The present invention relates to a finisher for use with a copier, printer or similar image forming apparatus and operable to staple or otherwise finish paper sheets which are sequentially driven out of the apparatus.
An image forming apparatus may be operated with a finisher which has a shift tray and loads the tray with paper sheets sequentially coming out of the apparatus while physically shifting them stack by stack. Specifically, the shift tray or discharge tray is driven by paper sorting means to move in a reciprocating motion in a horizontal plane and in a direction perpendicular to an intended direction of paper discharge while collecting the incoming paper sheets. A paper sheet driven out onto the tray is positioned in the perpendicular direction by a positioning member, and then the discharge tray is moved. A drive source and a drive transmission line for so moving the discharge tray are ususally mounted in such a manner as to be movable integrally with the tray.
A problem with the prior art finisher described above is that when the discharge tray is moved, the positioning member is apt to interfere with the paper sheets having been sorted and stacked on the tray and to thereby dislocate the paper stack while the tray is shifted. Further, it is likely that the paper sheet cannot be positioned not only in the shifting direction but also in the direction perpendicular thereto. While the tray may be shifted while being moved up and down to eliminate to above problem, as proposed in the past, such a movement is not practicable without resorting to a complicated mechanism and degrades the reliability of operation.
The drive source and drive transmission line associated with the discharge tray are movable integrally with the tray, as stated above. This is undesirable in that both the drive mechanism itself and the associated power supply line are complicated, and in that the load acting against the shift is too heavy to allow the tray to move at high speed.
Another drawback with the prior art finisher is that as the number of paper sheets sequentially driven out onto the discharge tray and, therefore, the height of the paper stack on the tray increases, the paper driving motion and the shifting motion fail to accurately match each other. Then, the paper sheets are dislocated by the shift of the tray resulting in the the sorting operations being disturbed. Such dislocation of paper sheets often occurs not only in the shifting direction but also in the direction perpendicular thereto. While an exclusive mechanism for sensing the height of a part of the positioning member may be used, it complicates the mechanism adapted to eliminate the interference of the positioning member and lacks reliability of operation.